// #define BOOST_REGEX_MATCH_EXTRA 
#include <string>
#include <iostream>
#include <boost/regex.hpp>

boost::regex e("(\\d{3,4})[- ]?(\\d{4})[- ]?(\\d{4})[- ]?(\\d{4})");

bool is_possible_card_number(const std::string& input)
{
	//
	// return false for partial match, true for full match, or throw for
	// impossible match based on what we have so far...
	boost::match_results<std::string::const_iterator> what;
	if(0 == boost::regex_match(input, what, e, boost::match_default | boost::match_partial))
	{
		// the input so far could not possibly be valid so reject it:
		throw std::runtime_error(
			"Invalid data entered - this could not possibly be a valid card number");
	}
	// OK so far so good, but have we finished?
	if(what[0].matched)
	{
		// excellent, we have a result:
		return true;
	}
	// what we have so far is only a partial match...
	return false;
}